1. Field of the Invention
The present invention generally relates to a piezoelectric thin-film resonator that utilizes the conversion between an electrical signal and a bulk acoustic wave, and more particularly, to a filter using the piezoelectric thin-film resonator.
2. Description of the Related Art
As wireless communication devices such as mobile-phone handsets have widely spread, there is an increasing demand for compact and light resonators and filters having these resonators. Conventionally, dielectric filters and surface acoustic wave (SAW) filters are mainly used. Recently, attention has been drawn to piezoelectric thin-film resonators and filters using these resonators. The piezoelectric thin-film resonators have good high-frequency characteristics and are capable of realizing compact and monolithic devices.
A film bulk acoustic resonator (FBAR) is known as one of the piezoelectric thin-film resonators. The FBAR has a device substrate on which a laminate structure (composed of multiple films). The laminate structure has, as essential structural elements, an upper electrode, a piezoelectric film, and a lower electrode. An opening (via hole or cavity) is formed below the lower electrode and is located below a region in which the upper electrode and the lower electrode face overlap with each other. The opening may be formed by etching (wet etching or dry etching) from the backside of a silicon substrate used as the device substrate, or by wet etching a sacrificed layer on the silicon substrate.
A high-frequency signal is applied between the upper electrode and the lower electrode, an acoustic wave is generated within the piezoelectric film sandwiched between the upper and lower electrodes. The acoustic wave thus generated is excited by the reverse piezoelectric effect and distortion arising from the piezoelectric effect. The acoustic wave is totally reflected by the surface of the upper electrode (film) that is in contact with air and the surface of the lower electrode (film) that is in contact with air. Thus, the acoustic wave is a thickness-extensional wave having main displacements in the thickness direction. In the present device structure, a resonance takes place at a frequency at which the total thickness H of the thin-film laminate structure composed of the upper electrode/piezoelectric film/lower electrode located above the opening is equal to an integer multiple of the ½ wavelength of the acoustic wave. The propagation velocity V of the acoustic wave is determined by the material used, and the resonant frequency F is written as F=nV/2H. By utilizing the resonant phenomenon, it is possible to control the resonant frequency with the thickness being used as a parameter for control and to realize the resonators and filters having desired frequency responses.
The upper and lower electrodes may be a film laminate made of a metal such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), or titanium (Ti), or an arbitrary combination of these metals. The piezoelectric film may be aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), or lead titanate (PbTiO3). Preferably, the piezoelectric film is aluminum nitride or zinc oxide having the main axis in the (002) direction. The device substrate may be made of silicon, glass or gallium arsenide (GaAs).
As described above, the piezoelectric thin-film resonator having the above-mentioned structure is required to have a via hole or cavity just below the lower electrode (or a dielectric film). In the following description, the via hole is defined as a hole that penetrates the device substrate and connect the upper and lower surfaces thereof, and the cavity is defined as a hole provided in the vicinity of the surface or just below the lower electrode.
FIG. 1 is a cross-sectional view of a conventional piezoelectric thin-film resonator described in Electron. Lett., 1981, vol. 17, pp 507-509 (hereinafter referred to as Document 1). A laminate structure is provided on a (100) silicon substrate 11 having a thermal oxide film (SiO2), and is composed of a lower electrode 13 that is an Au—Cr film, a piezoelectric film 14 that is a ZnO film, and an upper electrode 15 that is an aluminum film. A via hole 16 is formed below the film laminate. The via hole 16 may be formed by anisotropic etching using KOH or EDP (ethylenediamine and pyrocatechol) from the backside of the (100) silicon substrate 11.
In contrast, the piezoelectric thin-film resonator of the cavity type has, on a sacrificed layer, a laminate structure composed of an upper electrode, a piezoelectric film and a lower electrode (dielectric film as necessary). The sacrificed layer is removed by etching, so that the cavity can be defined. FIG. 2 is a cross-sectional view of a conventional piezoelectric thin-film resonator of the cavity type disclosed in Japanese Patent Application Publication No. 60-189307 (Document 2). A film laminate is composed of a lower electrode 23, a piezoelectric film 24 and an upper electrode 25 provided on a device substrate 21 having a thermal oxide film (SiO2) 22. A cavity 26 is defined below the film laminate. The cavity 26 may be formed by patterning an island-like sacrificed layer of ZnO, forming the film laminate on the sacrificed layer, and removing the sacrificed layer located below the film laminate by acid.
Japanese Patent Application Publication No. 2000-69594 (Document 3) shows another type of piezoelectric thin-film resonator, which is illustrated in FIG. 3. A film laminate composed of a lower electrode 33, a piezoelectric film 34 and an upper electrode 35 is provided on a silicon substrate 31 having a thermal oxide film (SiO2) 32. A cavity 36 is formed below the film laminate. This piezoelectric thin-film resonator may be fabricated as follows. A recess is formed in a part of the surface of the silicon substrate 31 by etching. Next, the thermal oxide film (SiO2) 32 is formed on the silicon substrate 31 in order to prevent phosphorus of PSG (PhosphoSilicate Glass) used as the sacrificed layer. Then, the sacrificed layer is deposited and is grinded and cleaned. Thereafter, the surface of the sacrificed layer is processed so as to have a mirror surface. Subsequently, the lower electrode 33, the piezoelectric film 34 and the upper electrode 35 are grown in this order. Finally, the PSG film is removed.
In the above-mentioned piezoelectric thin-film resonators, attention is usually drawn to the resonator structure in the thickness direction of the device substrate, namely, the membrane structure having the piezoelectric film sandwiched between the upper and lower electrodes, and the reflection structure for the acoustic wave by the lower electrode, namely, the structure of the lower electrode. That is, the electrode shape other than the membrane structure is not specifically considered.